In the climactic scene of Ray Bradbury’s motion picture script of Herman Melville’s classic novel Moby Dick, Captain Ahab (played by Gregory Peck) is at the bow of a longboat, chasing the white whale. Cunning and dangerous, Moby Dick dives deep. The rowers peer anxiously about them, looking for signs of the dreaded killer. Ahab commands his men to concentrate on their oars and not to look up.
“I’ll do your looking for you,” says Ahab.
In a sense, that is what great artists do. They do our looking for us. They perceive unities and beauties that we miss. Their talented eyes and minds see the world in better detail than most of us do, and their trained hands reproduce a kind of beauty that we might never have noticed.
This chapter deals with the way artists have used light in their works. I will concentrate mainly on painting because it is the art form that delights me most and because the painters faced not only the problems of form and color, but the unique problem of trying to present a convincing picture of the three-dimensional world on a flat, two-dimensional surface. Over the centuries, the painters succeeded so well in solving these problems that they have gone beyond pictorial representation of the world we can see and into abstractions that have little connection with the physical world around us.
Great visual art, be it painting or sculpture or photography, begins with the eyes. All the great artists knew this.
The visual arts began in the snows and cold of the Ice Age, when early humans first started to decorate their world. They shaped stones into rough figures of animals and pregnant women. They scratched primitive drawings onto the animal bones they used as tools. Fossilized bones and sticks have been found to bear markings that were not pictures, but marks that probably represent the first attempts at counting and arithmetic. Art and science were never far from one another in those early days. As we will see, even as late as the Renaissance, art and science progressed hand in hand. It took our modern pseudosophistication to separate the two.
Painting has been an important art form for at least thirty thousand years. The earliest evidence comes from cave dwellers who drew the outlines of their hands on rocks and colored them in. They drew meandering lines of charcoal inside their caves. Decoration? The innate human desire to “make one’s mark”? The earliest graffiti?
Cave-dwelling Cro-Magnon hunters painted on the walls of their caverns hauntingly graceful pictures of bison, horses, lions, gazelles, and even animals that are now extinct, such as the aurochs and woolly mammoth. These lovely and powerful paintings, which have been dated at some twenty thousand to thirty thousand years old, have been found chiefly in the southwestern portion of France and northeastern Spain, where the climate has allowed them to survive. But natural disasters are not the only threat to such treasures. Caves such as Lascaux and Altamira have been closed to the public to protect the paintings against the ravages of modern tourism.
The colors these early artists used came from the materials around them. Soil rich in iron oxides provided reds and browns.
Manganese oxide produced blue, iron carbonate yielded yellow. Charred wood from their fires gave them black. Chalk or ground seashells made white. Later on, as the ice retreated and vegetation became more abundant, crushed berries, roots, and leaves yielded a broader palette of colors.
The cave dwellers went to considerable effort to produce their art. The pictures they drew were not spur-of-the-moment doodles. They developed tools and paints purposefully and must have devoted much time and trouble to make their paintings. Cro-Magnon artists ground the natural pigments they found into powders and mixed them with water. In some locations, they heated the pigments, which changed the natural colors into new tones. Naturally occurring magnetite, for example, is black, but when heated it combines with oxygen from the air and turns into red hematite.
Archaeologists have found painting utensils at the cave sites: hollow bone receptacles (such as skulls) for holding the paints, bone mortars and pestles for grinding the pigments, even stone lamps for lighting the dark recesses of the caves.
By the time of the Middle Ages, the artist’s palette had gained a much richer variety of colors, although their sources were still much the same as before: natural pigments from colored earths and vegetable squeezings.
In medieval Europe a different art form arose in which the artists literally painted with light. I refer to the incredibly beautiful stained-glass windows of the great Gothic cathedrals.
Cathedrals such as those in Chartres, Cologne, and Amiens, and Notre Dame in Paris are among the finest architectural feats of the human mind. The techniques of Gothic architecture allowed stone structures to be built as delicately as lacework and to soar heavenward as no buildings could before. The weight of these magnificent edifices rested on stone pillars and buttresses; walls were not needed to hold the structure together. Solid stone walls could be replaced by windows to bring light into the church.
And what windows! In the twelfth and early thirteenth centuries, unknown European glassmakers and artists produced stained-glass windows of breathtaking beauty. Nothing like them has been produced since, although stained glass has enjoyed several revivals in popularity up to our own age. The glass made in those medieval decades was crude by comparison to modern glassmaking, yet its very crudeness added to the artistic effect of the finished windows. The glassmakers could not produce subtle colors. They added metallic oxides to their glass while it was in the molten state: copper for ruby red, cobalt for blue, manganese for purple, antimony for yellow, iron for green. The result was pure, rich, deep, bold colors that still strike the eye today with power and glory. The glass itself was uneven in thickness, rough by today’s refined standards. That gave it a texture and depth of color; it was what artists call a happy accident.
What sets stained glass aside from other forms of visual art is that light itself orchestrates the finished work. A stained-glass window is not a static picture. As the day progresses, the different angles and intensities of sunlight illuminate and alter the window, charging the vibrant colors with energy, toning them down as the hours pass. To see these masterworks in all their splendor, you should spend an entire day in one of the magnificent cathedrals and watch how the shifting sunlight interacts with the colors and forms of the glass.
In a sense, the art of stained glass suffered from growing refinements. The artists learned how to make smoother glass and subtler colors. The power and glory of the Age of Faith gave way to new interests, new conflicts. Over the centuries, artists often have returned to the medium of stained glass, but never with the simplicity and sheer visual impact of the earlier works. About a century ago, Louis Comfort Tiffany produced stained-glass lampshades and other household objects that are much prized by collectors and antique hunters. Yet the stained-glass windows of the great Gothic cathedrals stand alone in their power and beauty to this day.
Over the centuries, artists and artisans have developed rich palettes of colors for the purposes of painting, printing, dying, and otherwise beautifying the objects that we use every day of our lives.
Generally speaking, there are two types of coloring agents: dyes and paint pigments. Paint pigments are usually powders that are suspended in a medium such as oil. When paint is applied to a surface, the oil dries and hardens, leaving the microscopic particles of pigment scattered throughout it. The colors that we see come from light that strikes the paint and is scattered back toward our eyes by the particles of pigment.
When I was in kindergarten at the Abraham S. Jenks elementary school in south Philadelphia, we were given watercolors with which to paint. I watched the teacher mix red and blue to produce purple. I watched her mix red and yellow to produce orange. Yet when I mixed the colors from my little box of paints, I usually got nothing but a drab gray. Once in a while I could achieve a fairly ugly shade of brown. Watercolors defeated me.
Only much later in life did I learn that the main reason we were given watercolors was that they were cheap. Ours was not a well-to-do school district. There’s no telling how many potential Rembrandts were turned off painting in kindergarten by the watercolor experience. To this day, although I can sketch cartoons as well as most doodlers, the thought of painting a picture gives me the shudders.
Watercolors consist of a pigment mixed with an adhesive substance. When you add water, it forms a colored liquid that can be spread on a surface. The surface should be high-grade paper or plaster-neither of which was provided at dear old A.S. Jenks. When you apply watercolor to paper, the water quickly evaporates, leaving the pigment attached nicely to the paper. But there is no protection for the pigment. If you get it wet again, it will flow again. That was my big problem. My blue sky kept bleeding into my green meadow. The result: gray on gray.
About 2000 B.C., the Egyptians figured out a better way. Tempera paint added the yolk of an egg and a few other ingredients to the pigment and produced a hardy product that was easy to apply and dried quickly to a tough finish. The egg yolk, a mixture of a fatty oil, sticky albumin, and water, served to dissolve the pigment into very fine particles. Its faint yellowish color soon evaporated, leaving the pigment nicely embedded in the dried albumin, and it stuck to the surface being painted like dried egg yolk sticks to a dish.
Some modern painters still use egg tempera. The medium is not without its hazards, though. Robert Vickrey, renowned for his semisurrealistic pictures, once had a painting ruined by a dog that lapped up the eggy feast it found when a thoughtless gallery employee left one of Vickrey’s paintings on the floor of the gallery’s workroom.
By the fifteenth century, oil paint became the dominant medium for artists. The pigment is mixed with (usually) linseed oil. While the Flemish brothers Van Eyck are often credited with inventing oil painting, the true origin of oil paints is undoubtedly centuries older.
Oil paints are much easier to use than tempera (I will remain discreetly silent about watercolors), although the hardened oil does tend to darken over years of time. Many of Rembrandt’s paintings, for example, turned out to be much brighter and richer than previously thought once they had been cleaned of the oil film that had darkened over them.
Modern acrylic and latex paints use a plastic binder in place of oil or egg yolk. Plastic binders are easy to use and quite durable. However, some artists believe they lack the subtleness of color that is possible with oils.
While paint pigments are suspended in a binder such as oil or plastic, dyes are color materials that are usually dissolved in a solvent. Dyes can be dissolved in gelatin or clear plastic to make the colored filters used in stage lighting and photography. Color film is based on such dyes. Dyes that are soluble in water or other solvents are used for coloring fabrics and paper.
There is actually a third coloring agent called lakes, which are pigments that consist of particles that have been dyed. Lakes are used extensively in printers’ inks, including the ink of this book.
Dyeing has been an industry since before the beginning of written history. There was money to be made from producing colored cloth. The Chinese were wearing dyed clothing as early as 3000 B.C. By 2500 B.C., India was producing red dye from the root of the madder plant and blue from indigo plants. The Egyptians also were dying their linen clothing in greens, reds, and saffron yellow from dried crocuses.
By the time of the Roman Empire, the eastern Mediterranean city of Tyre had a monopoly on a dye that came to be known as royal purple. Obtained from a certain species of sea snail and from other shellfish, Tyrian purple was so expensive that only the very wealthiest and most powerful people could afford it. Thus it came to be associated with royalty. Only the ruling Roman families could wear togas edged with Tyrian purple. To this day, we associate the color with royalty.
Dyes were produced from natural sources down through all the centuries until 1856, when a seventeen-year-old English chemistry student, William Perkin, discovered that a substance obtained from the black sludge of coal tar could, when mixed with alcohol, dye silk purple. This first synthetic dye was colorfast, too, meaning that it did not wash out easily. Perkin did what any modern student would do. He dropped out of school, borrowed money from his father, and developed the first artificial dye process. He became very rich, was knighted, and before he died (at the age of sixty-nine), he developed the first artificial perfume, also from coal tar.
Perkin’s dye was the first of the so-called aniline dyes, aniline being the stuff from the coal tar from which the dye was derived. He called his dye aniline purple, but the more romantic French called it mauve, after their word for the purplish mallow flower. Mauve struck the haut monde world of Europe like a thunderclap, and soon no fashionable lady would be seen without the color adorning her ensemble. The Mauve Decade symbolized the late nineteenth-century Belle Epoque world of Paris.
More than that, though, Perkin’s work opened the door to the entire organic chemistry industry-which eventually led to plastics, pharmaceuticals, artificial fertilizers, and more.
Today dyes are still made principally from coal tar products or from petrochemicals, which come from petroleum.
And good old indigo dye, one of the earliest known to humankind, is still very much with us. It is the dye that makes blue jeans blue. However, if the jeans were completely dyed with indigo they would look more like a pair of walking neon bright lights than a pair of trousers. They would be garishly, embarrassingly blue. So blue jeans are only half dyed. The vertical warp threads are dyed with indigo, but the horizontal weft threads are left white. The result: blue jeans are popular around the world, especially when they’ve been washed or otherwise treated to tone down their original brightness even farther.
But we have gotten ahead of ourselves. Back to painting.
Over the centuries, artists acquired an empirical, trial-and-error knowledge of color, its uses, and how human beings perceive colors. The scientific study of color perception began only a scant few centuries ago. It is no accident of history that art and science flowered together during the Renaissance in Europe. As artists strove to understand the nature of color and how the human mind perceives colors, as they struggled to produce paintings on flat surfaces that gave the appearance of fully three-dimensional figures, they necessarily studied the physical nature of the eye and the brain and the physiology of vision.
Nor do the arts flourish separately. Painting, sculpture, and architecture seem to flower together at various periods of history.
Primitive men drew beautiful colored pictures on the walls of their caves. They also carved figures from bone and rock, and decorated their earthenware vases and pottery with colorful designs. They produced art, which art historian E.H. Gombrich defines as “anything done so superlatively well that we all but forget what the work is supposed to be, for sheer admiration of the way it is done.”
Over the millennia that followed, sculpture seemed to advance beyond painting. Sculptors were able to produce works of enormous beauty, power, and realism. The monumental statuary of ancient Egypt still awes the newcomer with its serene strength and majesty. The lifelike statues of Phidias and the other, unknown sculptors of ancient Athens are still marvels of grace and beauty. (Incidentally, they were never intended to be seen as they are today. The Athenians painted their sculptures; the gods’ beards were black, the goddesses’ robes were gold, their eyes were painted in, and perhaps even their skins were flesh-toned.)
But the painters of pictures could not portray fully rounded, three-dimensional figures on their flat boards and walls. Not even the best of the ancient painters could draw in perspective, as any grammar school student is taught to do today. No one knew how.
The laws of perspective were developed in the early Renaissance, when new interest in the heritage of knowledge left by the Greeks and Romans stirred European thinkers into new attitudes and discoveries.
Leonardo da Vinci is often credited with developing the laws of linear perspective, but he was not alone in achieving this feat. Filippo Brunelleschi, the great Florentine architect who died six years before Leonardo’s birth, is the true father of linear perspective. In The Story of Art, Gombrich writes: “It was Brunelleschi who gave the artists the mathematical means of solving this problem [of perspective]; and the excitement which this caused among his painter-friends must have been immense.”
While Euclid, around 300 B.C., understood that the size of an object as seen by the eye depends both on its actual size and its distance, it took another sixteen centuries for Europeans to develop the mathematics and perform the physical experiments that solidified the laws of perspective. Artists developed an apparatus that has come to be called a Leonardo box, a device for physically showing how an object should be drawn from various points of view.
The Leonardo box allowed artists to learn how to draw an object that is foreshortened, that is, distorted because of the angle at which it is viewed. They would place the object—say, a lute—on a surface and then connect strings from various parts of it, through a frame that represented the picture’s frame, to a focal point that represented where the eye of the beholder would be. Then they measured exactly where each string passed the plane of the frame; so many inches from the top, so many inches from the left side (or the bottom and the right side). A dot could be placed on a sheet of paper to represent where that string crossed the plane. With enough strings and enough dots, the figure of the lute took shape on the paper as it would be seen by the eye viewing it from that angle.
v~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~v
A “Leonardo box” being used by Renaissance artists to develop the techniques of perspective.
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
Suddenly Florentines were gaping at pictures of such solidity and reality that they found it hard to believe they had been painted on a flat wall. Masaccio’s Holy Trinity, in the church of Santa Maria Novella in Florence, seemed to them as if a hole had been broken through the wall of the church to reveal the Crucifixion scene. The figures appeared as solid and threedimensional as a sculpture.
Perspective did indeed cause immense excitement among the painters. According to Gombrich, Paolo Ucello, a contemporary of Brunelleschi “spent nights and days drawing objects in foreshortening, and setting himself ever new problems.... He was so engrossed in these studies that he would hardly look up when his wife called him to go to bed, and would exclaim, ‘What a sweet thing perspective is!’”
Thus it was no accident of history that mathematics and the laws of linear perspective developed simultaneously in the Renaissance. The burgeoning of curiosity and invention brought new knowledge of the arts and the sciences simultaneously.
Leonardo, that troubled genius who was born forty years before Columbus discovered the New World, is the epitome of the Renaissance artist. Curious about all things, he delved into human anatomy despite the prohibitions of the Church, because he had to know how the body was built and how it worked in order to paint it properly. He was compelled to understand how human vision functioned in order to create pictures that showed what he wanted his audiences to see.
To this day, his Mona Lisa is the best-known painting on Earth. There is more than artistic genius behind Lisa Giaconda’s smile. There is the early prescientific understanding of color contrasts, of perspective, of shading, and of subtle psychological tricks such as the fantasy landscape in the background of the picture.
Leonardo worked alone. His voluminous notes were written in code and backward, so that they could be read only in a mirror. He was not a scientist in the sense of Newton because he did not share his knowledge with the world. He may have been interested in learning about nature, but he kept his knowledge to himself and showed the outside world only the results: paintings such as the magnificent Last Supper, and mechanical contrivances that have been lost over the course of the years. He was a solitary genius, and his secretive notebooks show that his imagination ranged far beyond the limits of his time.
But science is a collaborative endeavor. It is a cathedral of knowledge that is constantly being enlarged and strengthened, stone by stone, bit by bit, by each generation of workers. Science cannot advance in secrecy. Leonardo might have been the most brilliant scientist of them all, had he shared his thoughts more fully. As it happened, the organized investigation of light and color had to wait more than a hundred years after his death.
When scientists such as Newton and, later, Maxwell probed the nature of light and the basis for color, they eventually saw the connections between the phenomena of the world around us and the workings of the visual system inside our heads.
We have seen that the three types of cones in the retina are sensitive to red, blue, and green light. Scientific experiments, beginning around the time of Isaac Newton, laid a firm foundation of understanding about color vision and color mixing.
Each color that we perceive consists of light of certain wavelengths. For example, blue light is concentrated in the wavelengths between 450 and five hundred nanometers; green, about 525 to 575 nanometers; red, from roughly 650 nanometers out to beyond human perceptual range. It is possible to measure the actual power, in watts, of a beam of light by using a spectroscope, an instrument that can break up white light into its component colors, like a prism, and detect not only the wavelengths of incoming light, but also their energy content.
The spectroscope allows scientists to produce spectral power distribution (SPD) curves, graphs that plot the energy content of various colors against their wavelengths. Thus the spectroscope allows us to measure the physical properties of colors. From this knowledge we can predict how these colors will be perceived in the brain and how they can be mixed to produce all the various hues of the rainbow (see Plate 2). What painters did by hard-won trial and error, science has organized into a formula.
Though it is impossible to describe a color subjectively, colors can at least be defined objectively in terms of their wavelengths and SPD. As we saw earlier, you would have a hard time telling a friend over the telephone exactly what color your new coat is. But if you had access to a spectroscope, you could tell your friend the wavelengths of the coat’s color and the energy content of the light reflected from it. That might help your friend visualize its color—if your friend were a physicist.
Our perception of colors depends not only on the wavelength and energy content of the light our eyes receive, but on two other attributes as well. Psychological studies have shown that saturation and brightness have important effects on how the brain perceives the colors that the eye sees.
Colors come in various hues, or wavelengths (see Plate 5). In addition to its hue, though, a color has the property of saturation. In this sense, saturation means the lack of “whiteness” in the color, how much the color differs from white or gray. Pink, for example, is unsaturated red. If you are mixing paints, to make pink you would blend red with white. Harvard’s crimson is a highly saturated red; no need for blending white with it.
Brightness refers to the intensity of the color. Day-Glo orange, the color that school crossing guards adorn their uniforms with, can be seen by even the most myopic motorist from a good distance away. It is a very intense, very bright color. The noonday Sun is a very bright yellow, too bright to look at for more than a fleeting fraction of a second. When that same Sun has made its way to the western horizon, not only has its hue changed (due to atmospheric absorption), but its brightness has decreased.
Brightness has an effect on our perception of color. Increasing brightness makes reds appear more yellow and violets appear bluer. Colors at the red end of the spectrum, that is, with wavelengths longer than about 575 nanometers (green), appear to shift toward the blue as brightness increases; colors at the blue, short-wavelength end of the spectrum seem to shift toward the red. With decreasing illumination, they shift the other way. In other words, as brightness increases, colors seem to shift toward the middle range of the visible spectrum. As brightness decreases, reds become redder and blues bluer.
White light, however, remains pretty constant no matter the brightness of the illumination. As we shall shortly see, this brightness constancy forms the basis for one of the theories of color vision.
Colors produce emotional reactions in us. We tend to think of reddish colors as warm and of bluish colors as cool. Red, yellow, and orange usually are associated with excitement and passion. Blue and green are restful, calming hues. Purple often is identified with dignity or even sadness, while black, brown, and gray are the colors of melancholy and depression.
Our emotional responses to colors have given phrases to our everyday speech. We see red, feel blue, become green with envy, purple with rage, or sink into a brown study. A red light means Stop! Danger! A green light is the go-ahead signal. We associate black with death, white with purity, and blood red with anger.
Physiological studies have shown that colors can influence blood pressure, heartbeat, breathing rate, perspiration, and even brain wave patterns. However, these physical responses are usually transitory. And repeated exposure to the same color does not produce a repeated physical response. Familiarity breeds not contempt but ennui.
Newton and those who followed him brought organized sense to the rules of thumb that artists had used for centuries. In particular, the scientists uncovered the physical basis for color mixing.
Remember the problems I had with watercolors? Newton was smart enough to work with light, not water paints. Using monochromatic beams of light, individual colors coming out of his prisms, he found that if he mixed green and red light, he obtained yellow. Mixing blue and red produced magenta (purple).
Newton had hit upon the technique of additive color mixing (see Plate 3). That is, one light is added to another. The two different spectral power distributions add together to form an SPD that is the sum of the two original SPDs. Not only does the color change, but the brightness is increased as well.
Young showed that virtually all the colors we can perceive can be created by blends of blue, green, and red. Today we realize that these are the colors to which the cones of our retinas are sensitive. By mixing these primary colors, almost any other color can be produced. Mix all three primary colors together and what do you get? White, of course.
If two colors produce white when they are added together, they are called complementary. The complement of a primary color is called a secondary color. To illustrate, we saw a few paragraphs above that mixing blue and red yields magenta:
B + R → M
Blue plus green produces turquoise, more properly called cyan:
B + G → C
And mixing the three primaries—blue, green, and red—produces white:
B + G + R → W
Now then, if the complement of a color is the color that will produce white when it is added to the original, then the complement of red is cyan, since cyan is composed of blue plus green, and red mixed with blue and green yields white:
R + C → W
Cyan, then, is a secondary color and the complement of red.
To make sense out of additive color mixing, and to simplify the rules, Newton devised the first color wheel, a chart that shows the primary colors arranged in a circle around “basic white,” with the complementary colors placed diagonally opposite the primaries.
All well and good, as far as the colors of light are concerned, but what about my water paints?
What my kindergarten teacher did not tell me (possibly because she did not know) is that mixing paints is very different from mixing lights. When it comes to mixing paints, we enter the realm of subtractive color mixing (see Plate 4).
This is easier to understand if we begin with rays of light. Subtractive color mixing takes place when we pass a beam of light through a colored filter. The filter can be a piece of colored glass or plastic, even a container of liquid that is colored by a dye. The filter absorbs part of the light and makes it dimmer when it emerges on the other side.
It also changes the light’s color. The filter absorbs some of the wavelengths of the incoming light. It alters the light’s SPD, or spectral power distribution curves. In a sense, the filter plays the same role as a university’s entrance examinations. All sorts of students want to attend the university; they all take the exam. Only those with the necessary knowledge get past the exam and actually enter the university. They are “on the right wavelength” to become university students. The others are not allowed into the school. Similarly, a colored filter stops (absorbs) all the light that is not “on the right wavelength” to pass through it.
Filters are usually called by the color of the light they allow through them. A red filter, for example, absorbs all the wavelengths of white light except those down at the long end of the visible spectrum. A blue filter stops everything but the shortest wavelengths.
Take a beam of white light and pass it through two filters, one red and one blue. What comes through the second filter? Nothing. Nothing visible, that is. All the visible wavelengths have been absorbed by the two filters.
Subtractive color mixing is trickier than additive because you can use more than one filter. You can start with a beam of white light, for instance, and pass it through a magenta filter and then a yellow one. The magenta filter allows only blue and red to pass through. The yellow filter, by itself, would pass green and red, but since the magenta filter has already screened out the green light, only red gets through the yellow filter.
For subtractive color mixing, the primary colors are magenta, yellow, and cyan. Combinations of two or all three of these colors can produce all the colors of the spectrum out of white light. Recognize that magenta is actually a combination of blue and red; yellow is a combination of green and red; and cyan is a combination of blue and green. Magenta, yellow, and cyan are each products of the three basic colors to which our retinas are sensitive.
Notice, too, that magenta, yellow, and cyan are the complements of the three additive primaries: blue, green, and red.
This is more than arcane trivia for physicists. The rules of subtractive color mixing govern any and all processes involving colored materials-such as paints and inks. When Leonardo painted the Mona Lisa, and when Hugh Heffner pored over the color proofs of Playboy’s latest centerfold, the colors they saw depended on subtractive color mixing.
The colors in paints and inks are caused by pigments suspended in the fluid. Many pigments exist in nature. As we have seen, our ability to see colors depends on pigments in the cones of our eyes. Our ability to see at all depends on the pigments in our retinal rods and cones.
In printing, the pigment of the ink attaches chemically to the paper while the fluid portion of the ink evaporates. Touch the paper too soon after printing and the ink smears; you must wait for it to dry for the fluid to evaporate and the pigment’s chemical bonding to the paper to take hold.
Modern four-color printing is based on the subtractive primaries of magenta, yellow, and cyan, plus black. These colors are sprayed onto the paper as microscopic dots that partially overlap. Where they overlap, subtractive color mixing determines the color seen by our eyes. Where they do not overlap, the colors are determined by a modified form of additive color mixing.
In oil paints, the fluid is usually linseed oil, which dries on the canvas with the pigment particles and helps to provide a protective coat for them. The particles of pigment are suspended in the oil even after it dries; they do not lie flat on the canvas. Instead, they are “frozen” into the dried oil, suspended like raisins in a cake. When light strikes the painting, it is reflected off these suspended particles of pigment. This gives the feeling of texture to a painting, because the light striking the paint can be reflected into our eyes from various depths in the layers of suspended pigments. Great artists can make you almost feel the fabrics that the people are wearing, the glossiness of the glass the lady is holding, the softness of the little dog’s fur.
The light reflected from the paint particles takes on the color of the pigment following the laws of subtractive color mixing. Blue pigment, for example, absorbs all the incident white light except the blue wavelengths; those it reflects into our eyes.
Each pigment, therefore, absorbs some light and reflects some. The pigments act in much the same way as any filter, although now the light is bouncing off the pigment particles rather than passing through them. Every substance in the universe absorbs or reflects light; even the purest Baccarat crystal bounces a teeny portion of light off its surface while letting the rest of the light through. Nothing is totally transparent.
Scientists study the way light is reflected by objects and draw up reflectance curves that show which wavelengths, and what percentage of the incoming light, are reflected. Green plants absorb red and blue light pretty effectively and reflect light from the middle ranges of the spectrum; that is why they appear green.
Now we can see why some fluorescent lights can make even the loveliest complexion look ghastly. The colors we perceive depend on the spectral power distribution of the light and the reflectance of the lady’s skin. Even though the reflectance curve of the skin stays the same, if the SPD of the illuminating light changes, our perception of the lady’s complexion will change too.
Additive and subtractive color mixing determine how we perceive colors. They are the basis of all painting and printing. In turn, color perception depends on the hue, saturation, and brightness of the colors we see. But there are other effects involved in color perception that are due to the “wiring” of the photoreceptors and nerve cells of the retina.
By the time the Renaissance was burgeoning in Europe, painters realized that they could play certain tricks on the eyes of their beholders. For example, the colors we perceive depend to a considerable extent on the brightness and saturation of the colors surrounding the one we are looking at. Moreover, the eye can be fooled by optical illusions that make two objects of the same size appear to be different sizes.
Most of these tricks depend on the phenomenon of contrast. The brain tends to judge colors and sizes, at least in part, by comparing them to the other objects in view. Hollywood learned this in the early years of the motion picture industry. When you want the hero of your western to be a strapping six-footer, but the actor playing the role is only five-three, you don’t stretch the actor, you lower the door frames. Western sets are built small so that the actors will look bigger. A standing joke around the studios is that John Wayne was only five-foot-six and rode a Shetland pony. (The Duke was actually well over six feet tall; no one ever had to downsize a set for him.)
You know from experience that your vision can adapt to low levels of light very quickly. Step into a movie theater that is so dark you can’t see the seats at all, and in a few moments your eyes will adapt and you will be able to find an empty seat without too much trouble.
Your vision can adapt just as quickly to increases in brightness. From time to time I am interviewed by television news reporters. Almost invariably, when the camera crew first turns on their powerful lights, my eyes squint and I feel something very close to physical pain. Yet in a few moments my eyes adjust and I can conduct the interview quite easily.
This is called lateral inhibition. The eye adjusts to increases in illumination in two main ways. First, the iris closes down, reducing the amount of light allowed into the eye. This can cut down the incoming light by a factor of sixteen. Second, the retina’s sensitivity to light is automatically regulated by the chemical reactions of the pigments in the rods and, in the case of bright light, particularly in the cones.
The sensitivity of one area of the retina is modified to some extent by the intensity of light falling on the retinal areas around it. This is called lateral brightness adaptation. For example, an area of gray will appear lighter when surrounded by white than it will when surrounded by black.
Such effects also apply to our perception of colors. Goethe’s Theory of Colors, published in 1810, gained notoriety for its attack on Newton’s work. Goethe cited many examples of color-contrast phenomena that he believed refuted Newton. They did not, but the examples did pose fascinating questions about how colors are perceived. One of Goethe’s examples was to light a candle at twilight and place it on a white sheet of paper, then hold a pencil upright so that the candle cast its shadow upon the paper. The shadow, with the dying rays of the Sun falling on it, will appear a beautiful blue. Explain that! Goethe challenged.
Newton could not (he was long dead), but later scientists did. It is a matter of lateral inhibition. The yellow candlelight stimulates the retina’s red- and green-sensitive cones preferentially. The shadow of the pencil is illuminated by white sunlight. But the sensitivity of the red and green cones has been reduced by lateral inhibition; it’s as if they are saying to the brain, “Hey, I’m already busy taking in this candlelight. I can’t handle more work right now.” What is left is the blue-sensitive cones; they take over the chore of sending information about the pencil’s shadow to the visual cortex, but the information is colored blue because that is the only signal those particular cones can send.
The basic theory of color vision was laid down by Young nearly two centuries ago and has been quantified by later scientists. Young’s trichromatic theory, the concept that all the colors we perceive are mixtures of blue, green, and red, is supported by the physiology of the retina with its cone cells that are sensitive to blue, green, and red.
But does that tell the entire story?
In 1878, the German physiologist and psychologist Ewald Hering asked an intriguing question: why are there certain mixtures of colors that we do not see? For example, we never see a yellow-blue mixture. Mix yellow and blue and you get either unsaturated yellow, unsaturated blue, or blah gray. (Remember, gray is what completely color-blind persons see.)
Hering claimed that there are actually four basic colors: blue, green, red, and yellow. How can only three color receptors in the eye deal with four pure colors? Hering tried to explain it by suggesting that there are actually four sets of color receptors in the eye and that they work in pairs that always are opposed to one another so that we perceive the colors as we do.
But the physiological evidence showed only three color receptors in the human eye. However, modern variations of Hering’s ideas propose that the nerve impulses from the retina do act in opposition to one another. Thus, if green and red receptors send signals of equal strength to the visual cortex, the cortex somehow cancels them out.
In the 1950s and subsequently, a new theory of color vision was proposed by Edwin Land. The inventor of the Polaroid camera, Land was particularly interested in developing a color film that would faithfully reproduce the colors that the human eye sees and the brain perceives. Land pointed out that the colors we perceive appear to be fairly constant despite the spectral power distribution of the light illuminating them. For example, a bed sheet appears white when it is hanging on a clothesline in the brilliant glare of the noonday Sun; it still appears white when it is softly illuminated by a single bedside lamp. Even skin tones illuminated by bluish fluorescent light are still perceived as skin tones; ghastly as they may appear, they are still recognizable.
Land suggested that this color constancy is an important clue to the fundamental mechanism of human color vision. He invented the retinex theory, coining the word from blending the words retina and cortex. As you might suspect, his theory involves the interaction between the retina and the visual cortex.
The retinex theory is fairly simple. Basically, it claims that the neural signals sent to the visual cortex from the retinal cells depend on the illumination of each region of the retina. The signals fired off by each individual nerve cell of the retina are compared in the visual cortex against one another on the basis of the light’s reflectance curve, not its SPD. According to Land, then, the colors we perceive depend more on the way surfaces reflect light than on the light illuminating those surfaces.
As you might suspect by now, the details of human color perceptions are still largely unknown. What is known is that the retina possesses three types of color receptors, sensitive to blue, green, and red. The nerve impulses sent from the retina to the visual cortex are extremely complex. Lateral inhibition, contrast effects, and color constancy all appear to play a role in the way we perceive colors.
Artists have unconsciously utilized the way we perceive light and color to produce their works. Unconsciously, at least, until modern times, when the scientific understandings of human vision have allowed artists to approach their work more knowledgeably.
Great artists, even those who came long before our modern understanding of human vision, used their own eyes and minds, their own observations of the world around them and of human behavior, to produce works of immortal beauty. Such artists think in color, as Vincent van Gogh did when he wrote to his brother Theo about his painting of his bedroom in Arles:
The walls are pale violet. The ground is of red tiles. The wood of the bed and chairs is the yellow of fresh butter, the sheets and pillows very light greenish lemon. The coverlet scarlet. The window green. The toilet-table orange, the basin blue. The doors lilac.... The broad lines of the furniture... must express absolute rest.
Scientists can explain the finished result of such thinking, to a degree. No one can explain exactly how the mind perceives those wonderful colors or the emotional impact they make on the viewer.
Yet, even while van Gogh was living in Arles, other Frenchmen were inventing a new way to capture images of the world around us. The scientific understandings of optics and chemistry led to photography.